Arsenic Sorption on Sulfide Minerals

References: 

(1)  Bostick, B. C.; S. Fendorf. 2002. Arsenite sorption on troilite (FeS) and pyrite (FeS2). Geochim. Cosmochim. Acta In Press.

(2)  Bostick, B. C.; S. Fendorf; B. A. Manning. 2002. Arsenite adsorption on galena (PbS) and sphalerite (ZnS). Geochim. Cosmochim. Acta In Press.

Overview of Arsenic in the Environment:  Arsenic contamination has resulted in widespread environmental problems in many areas including Taiwan and Bangladesh. Anthropogenic sources of arsenic contamination include sulfide ore smelting and disposal, and use in agricultural chemicals, wood preservation, and the burning of fossil fuels enriched in arsenic. Arsenic also enters the environment through natural mineral weathering, volcanic outgassing, and through processes such as acid mine drainage.  Sorption reactions mitigate the hazards of As by maintaining dissolved concentrations at low levels. Arsenic speciation has a profound effect on its sorption behavior. In well oxygenated natural waters, As(V) as arsenate, AsO₄^3-, is the dominant species. Arsenate may substitute in sulfate or phosphate minerals, and forms strong complexes on Fe and Al (hydr)oxides. Under slightly reducing conditions, As(V) is reduced to arsenite, H₃AsO₃, which sorbs less strongly to Al (hydr)oxides. Often the reductive dissolution of Fe(III) minerals precedes As reduction, releasing sorbed As(V) into solution. Arsenic can react with sulfide under highly reducing conditions to form orpiment (As₂S₃), arsenopyrite (FeAsS) or other sulfide phases.
                     The redox stratification in ocean sediments (Cutter, 1991; Sullivan and Aller, 1996) exemplifies the effects that arsenic speciation has on solution concentrations. Similar trends are observed in freshwater lakes and rivers. In oxidized surface sediments, iron (hydr)oxides scavenge arsenate from oxic water columns and become enriched in this element. As depth increases, the redox potential typically decreases, causing reductive dissolution of iron (hydr)oxides. The increase in porewater Fe(II) is accompanied by arsenic release. Iron concentrations decrease below the anoxic boundary by the formation of sulfide minerals. Arsenic concentrations are similarly reduced, presumably through association with iron sulfides or through the formation of neat sulfide minerals. Selective extractions have shown that As in anoxic zones is highly pyritized. Unfortunately, differentiating adsorption from precipitation is not possible with these techniques. Arsenic may be sequestered in the sediments as an arsenic sulfide solid; however, sorption on sulfide minerals may also occur. Arsenic sulfides have been identified in lake sediments (Soma et al., 1994), but the identity of the sulfide species could not be determined explicitly. Sulfosalt rims formed around sulfide mineral grains when reacted with As or Sb, also suggesting that As is retained by sulfide mineral surfaces. Research presented here investigates the potential role of such reactions in As(III) retention.

Results:  Arsenic was rapidly and strongly sorbed to sulfide minerals, reacting most strongly with iron sulfide minerals and somewhat less strongly with lead and zinc sulfides.  Sorption in both cases was described by a Langmuir isotherm indicative of adsorption processes.  Interestingly, arsenite was most strongly retained at high pH (>7), somewhat different than for oxides, which retain As(III) most effectively under neutral and slightly acidic pH.  For As retained on ZnS and PbS, no change in As oxidaiton state resulted from sorption (as detected by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy).  Further investigation into the structure of adsorbed arsenite showed that As(III) was forming clusters on the surface, even at surface adsorbed concentrations well below a monolayer.  These clusters had the same As-S and As-As distances as thioarsenite solution complexes thought to be a major solution species of arsenite in sulfidic waters.  For As(III) sorbed on ZnS, this was confirmed by calculating the theoretical spectrum for the As₃S₃(SH)₃ trimer; similar but distinct complexes were formed on PbS.

       Arsenite sorbed to iron sulfides through a distinct mechanism.  Arsenic also adsorbed strongly; however, adsorption reactions were not found to be important for As retention.  Rather, surface precipitation was observed.  This surface precipitate is reduced relative to As(III), in fact, it has a similar oxidation state to zero-valent As as in FeAsS.  This surface precipitate was identified as an FeAsS-like phase based on these XANES results, and other EXAFS and XPS data.  These data indicate that As(III) oxidizes surface of iron sulfides, producing a reduced and relatively unstable arsenopyrite phase, and iron oxides.  Since these compounds appear to be quite reactive, arsenic retained on sulfides will likely be easily released when environmental conditions favor oxidation.

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